The invention relates to a method for operation of a compressor supplied by a power converter.
Under normal circumstances, the power consumption, torque and rotational speed of compressors, for example centrifugal compressors, turbocompressors, follow fixed proportionalities. The profile of these parameters can be described using families of characteristics. In particular, a diagram of pressure against volumetric flow is used to illustrate an operating state of the compressor. FIG. 1 illustrates such an H-Q diagram. Under normal conditions, the compressor is operated only in a permissible region of the family of characteristics. The region is permissible with the exception of the hatched region SB of this family of characteristics. The hatched region SB marks an impermissible region SB for the compressor which must be avoided without fail. If a compressor is operated in this impermissible region SB, this may result in the compressor being damaged or destroyed. Entry into this impermissible region SB is defined by a so-called pump limit characteristic curve SG which is also referred to as a surge limit. If a compressor enters this impermissible region SB for whatever reasons, the “pumping” fault, also referred to as surge, is present.
The “pumping” fault (surge) involves the pressure of the compressor exceeding a particular value that depends on the volumetric flow Q. That is to say, in the H-Q diagram shown in FIG. 1, the pump limit characteristic curve SG is crossed, with the result that the operating point of the compressor is in the hatched region SG in the event of a fault. Flow separation at the compressor blades results in the case of such a fault. The result is that the differential pressure H collapses, as a result of which part of the volumetric flow Q flows backward through the compressor. However, the reduction in the differential pressure H also means that the impermissible region SB for the compressor is left again. However, the same conditions as before the fault are thus attained and the process is repeated cyclically. The compressor pumps as a result of changing back and forth between the permissible region and the impermissible region. The result of this pumping is that the compressor blades begin to vibrate. This considerably increases the risk of a blade breaking. In addition, this pumping of the compressor results in great pressure fluctuations in the pipe system connected to the compressor. The surge limit SG which characterizes the limit characteristic curve of the permissible region is dependent on the gas density and, with a given compressor material, on the temperature of the gas.
In order to be able to determine this “pumping” fault, operating parameters such as pressure difference H, volumetric flow Q, gas density GD or gas temperature GT have hitherto been measured. These parameters are supplied to a compressor controller which uses them to determine an operating point in the H-Q diagram. A predetermined pump limit characteristic curve (“surge limit”) is stored in this compressor controller. When this pump limit characteristic curve is approached in the case of compressors having a constant rotational speed, the differential pressure H is reduced using a mechanical actuator at the compressor inlet. Such a mechanical actuator is an inlet guide blade apparatus having an actuating time in the range of seconds. In the case of compressors having a variable rotational speed, the rotational speed is decreased.
One disadvantage of this previous method is that the “pumping” fault is detected only when flow separation has already started at the compressor blades. That is to say, damage to the blades has already begun. In the case of a multistage compressor, the entire volumetric flow Q and the entire differential pressure H first of all change only slightly, with the result that the state is detected only when it is highly pronounced. In the case of a single-stage compressor, the undesirable operating point occurs suddenly, with the result that conventional detection and control cannot react quickly enough. This results in the compressor having to be completely overhauled after a predetermined limited number of faults. Therefore, despite complicated metrology and control technology, it is only possible to determine that a “pumping” fault has occurred. Timely detection and a countermeasure resulting therefrom are not possible.